PROCESSOR TECHNOLOGY CORPORATION
Sol THEORY OF OPERATION SECTION VIII
causes LED3, the LOCAL light, to turn on. A second closure of the
LOCAL key toggles this section of U15 to the opposite condition.
Note that LOCAL has no affect on keyboard data.
The other outputs from U12 are BREAK (pin 12), CONTROL (pin
6) and REPEAT (pin 9). BREAK is inverted in U23 to become !BRK on
pin 4 of J1. CONTROL is applied directly to input pin 1 of U18 so
that the control character related to the low order bits enters U1
and U2.
REPEAT is applied to pins 10 and 11 of NAND gate U27 and pin
13 of NAND gate U16. The input to U27 is gated with UPPER_CASE to
generate !RST at pin 13 of J1. This means, of course, that REPEAT and
UPPER_CASE must be depressed at the same time to generate !RST.
On pin 13 of U16, REPEAT enables that gate so that U16 trans-
mits the output on pin 9 of U9. U9 is connected as a two-stage shift
register whose input (pin 2) is ground. It is clocked by clock_128
from U5.
U9 is initially set with output pins 5 and 9 high during the
third count cycle by PKD_minus_1. This is also the time when U12
outputs data. If the key is released, U9 clears to a low on pin 9
five count cycles following KEY. If the key is held down, U9 cannot
shift since PKD minus I remains on preset input pins 4 and 10.
When REPEAT exists at pin 13 of U16, pin 11 of U16 is low to
inhibit U25 and U27 at pin 13. This prevents further KEY signals and
disables the n-key rollover circuitry. The low on pin 11 of U16 is
also inverted by open collector inverter U24 to enable the repeat
oscillator (timer U3, R4, R5 and C3). U3 generates a square wave on
pin 3 with a period determined by the RC network.
This clocks the first stage of D flip-flop U11, the !STROBE
generator, and U11 produces the previously discussed 6 usec !STROBE.
U11 continues to generate !STROBE at the repeat oscillator rate until
either the REPEAT or character key is released. And with each !STROBE,
of course, the data associated with the character key is latched into
U1 and U2.
Eight ASCII-coded data bits are output by U1 and U2 to J1 as
indicated. Seven bits (0-6) are used for ASCII characters, and the
eighth bit (7) is set only for certain control characters that are
recognized by the Sol program. These are used for control functions
such as MODE_SELECT and cursor movement.
The remaining circuit, R32 and C14, initializes the keyboard
when power is applied. That is, it resets the output latches and the
SHIFT/SHIFT_LOCK, UPPER_CASE and LOCAL flip-flops. It also inhibits
STROBE at pin 1 of NAND gate U10.
VIII-45
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